U.S. patent number 6,051,843 [Application Number 09/181,912] was granted by the patent office on 2000-04-18 for exposure apparatus and method which synchronously moves the mask and the substrate to measure displacement.
This patent grant is currently assigned to Nikon Corporation. Invention is credited to Kenji Nishi.
United States Patent |
6,051,843 |
Nishi |
April 18, 2000 |
Exposure apparatus and method which synchronously moves the mask
and the substrate to measure displacement
Abstract
There is disclosed an exposure method for transferring, using an
optical system for illuminating a mask having patterns to be
transferred on a substrate and a projection optical system for
projecting images of the patterns to the substrate, the patterns to
the substrate through the projection optical system by means of
scanning the mask and the substrate synchronously relative to the
projection optical system. The method comprises the steps of
providing a plurality of measuring marks on the mask formed along a
relative scanning direction, and providing a plurality of reference
marks formed on the stage corresponding to the measuring marks,
respectively, moving the mask and the substrate synchronously in
the relative scanning direction to measure successively a
displacement amount between the measuring marks on the mask and the
reference marks, and obtaining a correspondence relation between a
coordinate system on the mask and a coordinate system on the stage
according to the displacement amount.
Inventors: |
Nishi; Kenji (Yokohama,
JP) |
Assignee: |
Nikon Corporation (Tokyo,
JP)
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Family
ID: |
26377256 |
Appl.
No.: |
09/181,912 |
Filed: |
October 29, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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831770 |
Apr 2, 1997 |
5844247 |
Dec 1, 1998 |
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608086 |
Feb 28, 1996 |
5646413 |
Jul 8, 1997 |
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476912 |
Jun 7, 1995 |
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203037 |
Feb 28, 1994 |
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Foreign Application Priority Data
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Feb 26, 1993 [JP] |
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5-38077 |
Dec 28, 1993 [JP] |
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5-334759 |
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Current U.S.
Class: |
250/548; 355/55;
356/399 |
Current CPC
Class: |
G03F
7/70358 (20130101); G03F 9/7088 (20130101); G03F
9/7015 (20130101); G03F 7/70775 (20130101) |
Current International
Class: |
G03F
9/00 (20060101); G03F 7/20 (20060101); G01N
021/86 () |
Field of
Search: |
;250/201.2,548
;355/43,53,55 ;356/399-401 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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4-324923 |
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Nov 1992 |
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JP |
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5-217835 |
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Aug 1993 |
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JP |
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Primary Examiner: Allen; Stephone B.
Attorney, Agent or Firm: Vorys, Sater, Seymour and Pease
LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a division of application Ser. No. 08/831,770
filed Apr. 2, 1997, (now U.S. Pat. No. 5,844,247 issued Dec. 4,
1998) and a division of application Ser. No. 08/608,086 filed Feb.
28, 1996 (now U.S. Pat. No. 5,646,413 issued Jul. 8, 1997), and a
continuation of application Ser. No. 08/476,912 filed Jun. 7, 1995,
(abandoned) and a continuation of application Ser. No. 08/203,037
filed Feb. 28, 1994 (abandoned).
Claims
What is claimed is:
1. A scanning exposure method in which an object supported by a
member and an exposure beam are moved relatively in a first
direction during a scanning exposure, the method comprising:
detecting positional information in the first direction of the
supporting member using a corner cube type reflection member which
is formed on the supporting member and which is used with a first
interferometer device; and
detecting positional information in a second direction
perpendicular to the first direction of the supporting member using
a reflection surface which is substantially parallel to the first
direction on the supporting member and which is used with a second
interferometer device.
2. A scanning exposure method according to claim 1, wherein the
object includes a mask having a pattern.
3. A scanning exposure method according to claim 1, wherein said
exposure beam includes light.
4. A scanning exposure method according to claim 1, further
comprising:
controlling a position of said supporting member based on the
detected positional information in said first and second
directions.
5. A device which is produced by using a scanning exposure method
according to claim 1.
6. A method of making a scanning exposure apparatus in which an
object and an exposure beam are moved relatively in a first
direction during a scanning exposure, the method comprising:
providing a supporting member which is movable in the first
direction while supporting the object;
providing a corner cube type reflection member which is arranged on
the supporting member;
providing a first interferometer device, optically connected with
the corner cube reflection member, which detects positional
information in the first direction of the supporting member;
providing a reflection surface, on the supporting member, which is
substantially parallel to the first direction; and
providing a second interferometer device, optically connected with
the reflection surface, which detects positional information in a
second direction perpendicular to the first direction.
7. A method according to claim 6, wherein the object includes a
mask having a pattern.
8. A method according to claim 6, wherein said exposure beam
includes light.
9. A method according to claim 6, further comprising:
providing a controlling system, connected with the first and second
interferometer devices, which controls a position of the supporting
member based on the positional information detected by the first
and second interferometer devices.
10. A method according to claim 6, wherein a plurality of the
corner cube type reflection members are spaced by a predetermined
distance in said second direction and disposed on said supporting
member, and said first interferometer device obtains positional
information of said supporting member in said first direction from
each of said plurality of corner cube type reflection members.
11. A method according to claim 10, further comprising:
obtaining rotational information of said supporting member based on
said positional information of said supporting member in said first
direction obtained from each of said plurality of corner cube type
reflection members.
12. A method according to claim 6, wherein said supporting member
has a first driving mechanism for moving said object in said first
direction and a second driving mechanism for fine moving said
object in said first direction, in said second direction, and in a
rotational direction.
13. A method according to claim 12, wherein said first driving
mechanism has a first stage movable in said first direction, and
said second driving mechanism has a second stage fine movable in
said first direction, in said second direction and in a rotational
direction.
14. A device which is produced by a scanning exposure apparatus
made by using a method according to claim 6.
15. A scanning exposure apparatus in which an object and an
exposure beam are moved relatively in a first direction during a
scanning exposure, the apparatus comprising:
a supporting member which is movable in the first direction while
supporting the object;
a corner cube type reflection member which is arranged on the
supporting member;
a first interferometer device, optically connected with the corner
cube reflection member, which detects positional information in the
first direction of the supporting member;
a reflection surface, provided on the supporting member, which is
substantially parallel to the first direction; and
a second interferometer device, optically connected with the
reflection surface, which detects positional information in a
second direction perpendicular to the first direction.
16. A scanning exposure apparatus according to claim 15, wherein
the object includes a mask having a pattern.
17. A scanning exposure apparatus according to claim 15, wherein
said exposure beam includes light.
18. A scanning exposure apparatus according to claim 15, wherein a
plurality of the corner cube type reflection members are spaced by
a predetermined distance in said second direction and disposed on
said supporting member, and
said interferometer device obtains positional information of said
supporting member in said first direction from each of said
plurality of corner cube type reflection members.
19. A scanning exposure apparatus according to claim 18, wherein
rotational information of said supporting member is obtained based
on said positional information of said supporting member in said
first direction obtained from each of said plurality of corner cube
type reflection members.
20. A scanning exposure apparatus according to claim 15, further
comprising:
a controlling system, connected with the first and second
interferometer devices, which controls a position of the supporting
member based on the positional information detected by the first
and second interferometer devices.
21. A scanning exposure apparatus according to claim 20, wherein
said supporting member has a first driving mechanism for moving
said object in said first direction and a second driving mechanism
for fine moving said object in said first direction, in said second
direction and in a rotational direction.
22. A scanning exposure apparatus according to claim 21, wherein
said first driving mechanism has a first stage movable in said
first direction, and said second driving mechanism has a second
stage fine movable in said first direction, in said second
direction and in a rotational direction.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of exposing and an apparatus
therefor. More particularly, the present invention relates to an
exposure apparatus of, for example, a slit scanning exposure type
and an exposure method advantageously applicable to such
apparatus.
2. Related Background Art
Projection type exposure apparatus has been used in manufacturing
semiconductor devices, liquid crystal displays and thin-film
magnetic heads through a photolithography process, in which
patterns of a photomask or a reticle (hereinafter, referred
generally to as "reticle") are transferred to the surface of a
substrate (wafer, glass plate, etc.) coated with a photosensitive
coating.
The conventional projection type exposure apparatus commonly used
is a reduction projection type exposure apparatus (stepper) that
moves individual shots on the wafer successively to an exposing
field of a projection optical system to reproduce pattern images of
the reticle on the shots using a photographic step-and-repeat
process.
In the typical steppers, a wafer coordinate system is corresponded
to a reticle coordinate system (reticle alignment).
Some steppers comprise an alignment microscope of an off-axis type
provided at one side surface of the projection optical system to
detect a position of the alignment mark (wafer mark) formed as a
correspondence to each shot on the wafer. In such a case, the shot
on the wafer is determined within the exposing field of the
projection optical system according to the position of the
associated wafer mark detected on the alignment microscope.
Accordingly, so-called base line amount should be obtained
previously that represents a distance between a reference point
(such as an exposure center) within the exposing field of the
projection optical system and a reference point in an observing
field of the alignment microscope of the off-axis type.
The reticle alignment and the base line measurement are disclosed
in detail in, for example, Japanese Patent Application Laid-Open
No. 4-324923 (corresponding to U.S. patent application Ser. No.
872,750 (filed on Apr. 21, 1992)).
In recent years, fine patterns for the semiconductor devices
require resolution of the projection optical system to be improved.
To improve the resolution, exposing light may be shifted to a
shorter wavelength or alternatively, the number of openings of the
projection optical system may be increased. In any event, it has
been difficult to maintain an image quality (such as distortion and
image plate deformation) on the entire exposing field with a
predetermined accuracy when it is intended to ensure as same
exposing field as conventional arts. With this respect, the
projection type exposure apparatuses based on so-called slit
scanning exposure type have taken a favorable turn.
In the projection type exposure apparatus of the slit scanning
exposure type, the reticle and the wafer are scanned relative to
and synchronous with a rectangular or arc-shaped illumination field
(hereinafter, referred to as a "slit-shaped illumination field") to
transfer the patterns of the reticle on the wafer. The slit
scanning exposure type thus makes it possible to reduce the
exposing field of the projection optical system as compared with
the stepper type, provided that the reproduced patterns are equal
in area to those reproduced using the stepper type. This may
improve the accuracy of the image quality within the exposing
field. A six-inch size is dominant for the conventional reticles
while a one-fifth factor is dominant as the projection
magnification of the projection optical system. At the
magnification of one-fifth factor, the six-inch reticle may
sometimes be insufficient for recent circuit patterns of the
semiconductor device having the increased area. As a result, the
projection optical systems should so designed that the projection
magnification of the projection optical system is changed to, for
example, quarter factors. To comply with requirements for such
reproduced patterns having the increased area, the slit scanning
exposure type can advantageously be applied.
In this event, the alignment method based on the reticle and wafer
coordinate systems used in the conventional steppers may be
unfavorable when being applied to the projection type exposure
apparatus of the slit scanning type. The projection magnification
of quarter factors adversely affects the accuracy of the alignment
because the alignment becomes more sensitive to writing errors of
the circuit patterns on the reticle.
A technique has been suggested in the above mentioned U.S. patent
application Ser. No. 872,750 (filed on Apr. 21, 1992) to measure a
rotation angle of the reticle by means of measuring simultaneously
the amount of displacement of two or more measuring marks rather
than moving the wafer stages in the wafer. However, the idea of
measuring the rotation angle using the simultaneous measuring of
the measuring marks cannot be applied to scanning directions of the
projection type exposure apparatus of the slit scanning exposure
type. Thus, there is a disadvantage that it is impossible to
measure the rotation angle of the reticle and wafer coordinate
systems and orthogonal amount of the coordinates of these
coordinate systems.
As for the method of measuring the base line amount between the
reference position within the exposing field of the projection
optical system and the reference position of the alignment system
of the off-axis type, the conventional measuring method using a
pair or marks on the reticle in the stepper is disadvantageous,
when it is applied to the projection type exposure apparatus of the
slit scanning exposure type with no modification, in that the
writing error of the reticle significantly affects the
measurements.
SUMMARY OF THE INVENTION
With respect to these problems, the present invention is directed
to provide an exposure method and an exposure apparatus capable of
reducing affect of the writing error between the patterns on the
reticle (mask), allowing positive alignment of the reticle
coordinate system (mask coordinate system) and the wafer coordinate
system (substrate coordinate system) in the exposure apparatus of
the slit scanning exposure type.
In light of this, speed of operation may sometimes be considered to
be more important than the accuracy of alignment depending on the
process. With this respect, another object of the present invention
is to provide an exposure method and an exposure apparatus capable
of aligning the reticle coordinate system (mask coordinate system)
with the wafer coordinate system (substrate coordinate system) at a
higher throughput.
Yet another object of the present invention is to provide an
exposure method and an exposure apparatus capable of reducing
affect of the writing error between the patterns on the reticle
(mask), allowing positive measurement of the base line amount
between the reference point in the exposing field of the projection
optical system and the reference point of the alignment system in
the exposure apparatus of the slit scanning exposure type.
Still another object of the present invention is to provide an
exposure method and an exposure apparatus in which the alignment
between the reticle coordinate system (mask coordinate system) and
the wafer coordinate system (substrate coordinate system) as well
as the base line amount are obtained for exposure.
In a case where the base line measurement is performed for every
predetermined number of wafer replacements, speed of operation may
be considered to be more important than the accuracy of alignment.
At the same time, the reticle coordinate system (mask coordinate
system) is preferably aligned to the wafer coordinate system
(substrate coordinate system). With this respect, another object of
the present invention is to provide an exposure method and an
exposure apparatus capable of aligning the reticle coordinate
system (mask coordinate system) to the wafer coordinate system
(substrate coordinate system) and of measuring the base line
therefor at a higher throughput for every predetermined number of
wafer replacements.
It is yet another object of the present invention to provide an
exposure method and an exposure apparatus using a plurality of
measuring reticle marks to reduce affect of, for example, the
writing errors between patterns on the reticle (mask).
It is still another object of the present invention to provide an
exposure method and an exposure apparatus capable of aligning the
reticle coordinate system (mask coordinate system) to the wafer
coordinate system (substrate coordinate system) and of measuring
the base line therefor with a high accuracy in consideration of an
error component to a relative scanning direction of the mask to the
wafer.
In an exposure method according to a first aspect of the present
invention, it is provided with an exposure method of transferring,
using an optical system for illuminating a mask having patterns to
be transferred to a substrate and a projection optical system for
projecting images of the patterns to the substrate, the patterns on
the mask to the substrate on a stage through the projection optical
system by means of scanning the mask and the substrate
synchronously relative to the projection optical system, wherein
the method comprises the steps of: providing a plurality of
measuring marks on the mask formed along a relative scanning
direction, and providing a plurality of reference marks formed on
the stage corresponding to the measuring marks, respectively;
moving the mask and the substrate synchronously in the relative
scanning direction to measure successively a displacement amount
between the measuring marks on the mask and the reference marks;
and obtaining a correspondence relation between a coordinate system
on the mask and a coordinate system on the stage according to the
displacement amount.
In an exposure method according to a second aspect of the present
invention, it is provided with an exposure method of transferring,
using an exposure apparatus having an optical system for
illuminating a mask having patterns to be transferred to a
substrate, a mask stage for holding the mask, a substrate stage for
holding the substrate, a projection optical system for projecting
images of the patterns to the substrate, and an alignment system
having its detection center at a position away from the optical
axis of the projection optical system at a predetermined distance,
the patterns to the substrate through the projection optical system
by means of scanning the mask and the substrate synchronously
relative to the projection optical system, wherein the method
comprises the steps of providing a plurality of measuring marks
formed on the mask along a relative scanning direction; providing
first reference marks corresponding to a part of the measuring
marks and second reference marks corresponding to the first
reference marks, respectively, the first and the second reference
marks being formed on the stage, the second reference marks being
away from the first reference marks at a given distance that is
recognized previously; moving the mask to the relative scanning
direction with the second reference marks observed through the
alignment system to measure successively a displacement amount
between the measuring marks on the mask and the first reference
marks; and obtaining a distance between a reference point within an
exposing field of the projection optical system and the detection
center according to the displacement amount between the measuring
marks and the first reference marks, to a displacement amount of
the second reference marks observed through the alignment system,
and to the given distance previously recognized.
In an exposure method according to a third aspect of the present
invention, it is provided with an exposure method of transferring,
using an exposure apparatus having an optical system for
illuminating a mask having patterns to be transferred to a
substrate, a mask stage for holding the mask, a substrate stage for
holding the substrate, a projection optical system for projecting
images of the patterns to the substrate, and an alignment system
having its detection center at a position away from the optical
axis of the projection optical system at a predetermined distance,
the patterns to the substrate through the projection optical system
by means of scanning the mask and the substrate synchronously
relative to the projection optical system, wherein the method
comprises the steps of providing a plurality of measuring marks
formed on the mask along a relative scanning direction; providing
first reference marks corresponding to each of the measuring marks
and second reference marks corresponding to the first reference
marks, respectively, the first and the second reference marks being
formed on the stage, the second reference marks being away from the
first reference marks at a given distance that is recognized
previously; moving the mask and the substrate to a scanning
direction to measure successively a displacement amount between the
measuring marks on the mask and the first reference marks; moving
the mask and the substrate relatively to the scanning direction to
measure successively a displacement amount of the second reference
marks; and obtaining a distance between a reference point within an
exposing field of the projection optical system and the detection
center according to the displacement amount between the measuring
marks and the first reference marks, to a displacement amount of
the second reference marks observed through the alignment system,
and to the given distance previously recognized.
In an exposure method according to a fourth aspect of the present
invention, it is provided with an exposure method of transferring,
by means of illuminating an illumination area of a predetermined
shape using an illumination light to scan a mask and a substrate
synchronously relative to the illumination area of a predetermined
shape, patterns on the mask within the illumination area of the
predetermined shape through a projection optical system to the
substrate on a stage, wherein the method comprises, with a
plurality of measuring marks formed on the mask along a relative
scanning direction and reference marks formed on the stage
corresponding to the measuring marks, a first step for measuring a
displacement amount between a part of the measuring marks and the
reference marks corresponding to the part of the measuring marks,
respectively; a second step for moving the mask and the substrate
synchronously to the relative scanning direction to measure
successively a displacement amount between the measuring marks on
the mask and the reference marks corresponding to the measuring
marks; and a third step for selecting one of the first and the
second steps to obtain a corresponding relation between a
coordinate system on the mask and a coordinate system on the stage
according to the displacement amount between the measuring marks
and the reference marks, respectively, obtained at the selected
step.
In an exposure method according to a fifth aspect of the present
invention, it is provided with an exposure method of transferring,
using an exposure apparatus having an optical system for
illuminating an illumination area of a predetermined shape using an
illumination light, a mask stage for holding a mask provided with
patterns to be exposed, a substrate stage for holding a substrate,
a projection optical system for projecting the patterns on the mask
to the substrate, an alignment system having its detection center
at a position away from the optical axis of the projection optical
system at a predetermined position, the patterns on the mask in the
illumination area of the predetermined shape through the projection
optical system to the substrate by means of scanning the mask and
the substrate synchronously relative to the illumination area of
the predetermined shape, wherein the method comprises, with a
plurality of measuring marks formed on the mask along a relative
scanning direction and a plurality of first reference marks
corresponding to the measuring marks and second reference marks
corresponding to the first reference marks, the first and the
second reference marks being formed on the stage, the second
reference marks being away from the first reference mark at a given
distance that is recognized previously, a first step for measuring
a displacement amount between a part of the measuring marks on the
mask-and the first reference marks corresponding to the part of the
measuring marks, respectively, and measuring a displacement amount
between the second reference marks corresponding to the part of the
first reference mark; a second step for moving the mask and the
substrate in synchronism with the scanning direction to measure
successively a displacement amount between the measuring marks and
the first reference marks corresponding to the measuring marks,
respectively, and a displacement amount of the second reference
marks; a third step for selecting one of the first and the second
step; and a fourth step for obtaining a corresponding relation
between a coordinate system on the mask stage and a coordinate
system on the substrate stage and a distance between a reference
point within an exposing field of the projection optical system and
the detection center according to information obtained during the
step selected at the third step and a given distance previously
recognized.
In an exposure method according to a sixth aspect of the present
invention, it is provided with an exposure method of transferring,
using an exposure apparatus having an optical system for
illuminating an illumination area of a predetermined shape using an
illumination light, a mask stage for holding a mask provided with
patterns to be exposed, a substrate stage for holding a substrate,
a projection optical system for projecting the patterns on the mask
to the substrate, an alignment system having its detection center
at a position away from the optical axis of the projection optical
system at a predetermined position, the patterns on the mask in the
illumination area of the predetermined shape through the projection
optical system to the substrate by means of scanning the mask and
the substrate synchronously relative to the illumination area of
the predetermined shape, wherein the method comprises, with a
plurality of measuring marks formed on the mask along a relative
scanning direction and a plurality of first reference marks
corresponding to the measuring marks and second reference marks
corresponding to the first reference marks, the first and the
second reference marks being formed on the stage, the second
reference marks being away from the first reference mark at a given
distance that is recognized previously, for every replacement of
predetermined number of substrates, a step for measuring a
displacement amount between a part of the measuring marks on the
mask and the first reference marks corresponding to the part of the
measuring marks, respectively, and measuring a displacement amount
between the second reference marks corresponding to the part of the
first reference mark; and a step for obtaining a corresponding
relation between a coordinate system on the mask and a coordinate
system on the stage and a distance between a reference point within
an exposing field of the projection optical system and the
detection center according to a displacement amount between one
measuring mark and one first reference mark, to a displacement
amount of the second reference marks, and to the given distance
recognized previously.
According to the first exposure method of this invention, it is
possible to reduce affect of the writing error of the measuring
marks on the mask by means of obtaining a parameter (such as the
magnification, a scaling in the scanning direction, rotation,
degree of parallelism in the scanning direction, and offsets in an
X direction and a Y direction) for use in aligning the mask
coordinate system with the substrate coordinate system by using the
least square approximation by means of, finally, matching the
displacement obtained, for example, at each position of the
measuring mark on the mark.
According to the second exposure method, it is possible to measure
positively the base line amount or the distance between the
reference point of the projection optical system and the reference
point of the alignment system by means of reducing the writing
error of the measuring marks on the mask through measurement
regarding to the measuring marks on the mask side.
According to the third exposure method, a plurality-of first
reference marks are formed on a reference mark member with being
correspondent with the measuring marks, respectively, on the mask.
In addition, a plurality of second reference marks are formed at
such a distance that corresponds to the distance between the
reference point within the exposing field of the projection optical
system and the reference point of the alignment system from the
first reference marks. Accordingly, the base line amount can be
measured more positively because the balancing is made across the
reference marks.
According to the fourth exposure method, simple measuring steps
based on a quick mode are selected, which allows calculation of the
corresponding relation between the coordinate system on the mask
and the coordinate system on the stage at a higher throughput
depending on the necessities.
According to the fifth exposure method, simple measuring steps
based on a quick mode are selected, which allows calculation of the
corresponding relation between the coordinate system on the mask
and the coordinate system on the stage as well as the base line
amount at a higher throughput depending on the necessities.
According to the sixth exposure method, simple measuring steps
based on a quick mode are performed for every exposure of a
predetermined number of substrates, which allows calculation of the
corresponding relation between the coordinate system on the mask
and the coordinate system on the stage as well as the base line
amount at a higher throughput when many substrates are subjected to
exposure continuously through the scanning method.
In an exposure method according to a seventh aspect of the present
invention, it is provided with an exposure method for transferring,
by means of illuminating an illumination area of a predetermined
shape using an illumination light and scanning a mask and a
substrate synchronously relative to the illumination area of the
predetermined shape, patterns on the mask in the illumination area
of the predetermined shape to the substrate on a stage through a
projection optical system, wherein the method comprises, with a
plurality of measuring marks formed on the mask along a relative
scanning direction and a plurality of reference marks formed on the
stage corresponding to a part of the measuring marks, the steps of
moving the mask to the relative scanning direction to measure
successively a displacement amount between the measuring marks on
the mask and the reference marks; and obtaining a corresponding
relation between a coordinate system on the mask and a coordinate
system on the stage.
In an exposure method according to an eighth aspect of the present
invention, it is provided with an exposure method for transferring,
using an exposure apparatus having an optical system for
illuminating an illumination area of a predetermined shape using an
illumination light, a mask stage for holding a mask provided with
patterns to be exposed, a substrate stage for holding a substrate,
a projection optical system for projecting the patterns on the mask
to the substrate, an alignment system having its detection center
at a position away from the optical axis of the projection optical
system at a predetermined position, the patterns on the mask in the
illumination area of the predetermined shape through the projection
optical system to the substrate by means of scanning the mask and
the substrate synchronously relative to the illumination area of
the predetermined shape, wherein the method comprises the steps of
forming on the substrate stage a reference mark detectable by the
alignment system to measure a displacement amount of the reference
marks by the alignment system; entering a mark error of the mask;
and obtaining a corresponding relation between a coordinate system
on the mask stage and a coordinate system on the substrate stage,
and a distance between a reference pint within an exposing field of
the projection optical system and the detection center.
In an exposure apparatus according to a first aspect of the present
invention, it is provided with an exposure apparatus comprising a
mask stage for holding a mask provided with patterns to be
transferred; a substrate stage for holding a substrate; an optical
system for illuminating the mask using an illumination light; a
projection optical system for projecting images of the patterns on
the mask to the substrate; and a first mark detecting system for
detecting a mask mark formed at a predetermined position on the
mask within an exposing field of the projection optical system, the
exposure apparatus being for use in scanning the mask and the
substrate synchronously relative to the projection optical system
to transfer the patterns on the mask to the substrate through the
projection optical system, wherein the apparatus further comprises
a reference plate provided on the substrate stage, the reference
plate comprising a plurality of first reference marks detectable by
the first mark detecting system through the projection optical
system; a plurality of measuring marks provided on the mask, each
of the measuring marks being provided along a relative scanning
direction with being correspondent with the reference mark; a
driving control system for use in observing a part of the first
reference marks and a part of the measuring marks through the first
mark detecting system to move the mask stage and the substrate
stage to the relative scanning direction such that a displacement
amount between the measuring marks on the mask and the reference
marks is measured successively; and calculating means for
calculating a corresponding relation between a coordinate system on
the mask stage and a coordinate system on the substrate stage.
In an exposure apparatus according to a second aspect of the
present invention, it is provided with an exposure apparatus
comprising a mask stage for holding a mask provided with patterns
to be transferred; a substrate stage for holding a substrate; an
optical system for illuminating the mask using an illumination
light; a projection optical system for projecting images of the
patterns on the mask to the substrate; and a first mark detecting
system for detecting a mask mark formed at a predetermined position
on the mask within an exposing field of the projection optical
system, the apparatus being for use in scanning the mask and the
substrate synchronously relative to the projection optical system
to transfer the patterns on the mask to the substrate through the
projection optical system, wherein the apparatus further comprises
a reference plate provided on the substrate stage, the reference
plate comprising a plurality of first reference marks detectable by
the first mark detecting system through the projection system and
second reference marks provided with being away from the first
reference marks at a given distance that is recognized previously;
a plurality of measuring marks provided on the mask, each of the
measuring mask being provided along the relative scanning direction
with being correspondent with the first reference mark; a driving
control system for use in moving the mask stage and the substrate
stage to the relative scanning direction such that the part of the
measuring marks and the part of the reference marks are observed
through the first mark detecting system and a displace amounts are
measured successively between the measuring marks on the mask and
the reference marks as well as between the second reference marks
with one of a plurality of second reference marks being observed
through a second mark detecting system; and calculating means for
calculating a corresponding relation between a coordinate system on
the mask stage and a coordinate system on the substrate stage and a
distance between a reference point within an exposing field of the
projection optical system and the detection center according to the
displacement amounts measured.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a structural diagram showing a projection type exposure
apparatus to which an embodiment of a projection exposure method
according to the present invention is applicable;
FIG. 2 is comprised of FIGS. 2A and 2B showing flow charts
illustrating an exposure method and a base line amount check method
according to a first embodiment of the present invention;
FIG. 3 is a perspective view showing a reticle loader system;
FIG. 4A is a view for use in describing an arrangement of alignment
marks on a reticle;
FIG. 4B is a view for use in describing an arrangement of alignment
marks or the like in an area conjugated with an effective field of
a projection optical system;
FIG. 4C is an enlarged view of fine alignment marks 29A, 29B, 29C,
29D, 30A, 30B, 30C and 30D;
FIG. 5A is a view for use in describing how to align a reticle
roughly;
FIG. 5B is a view showing a reduced version of FIG. 5A;
FIGS. 6A, 6B, 6C, 6D, 6E and 6F are views showing waveforms of
image pick-up signals supplied from an image pick-up device during
a rough alignment of the reticle;
FIG. 7A is a plan view of a stage at a wafer side;
FIG. 7B is a plan view of a stage at a reticle side;
FIG. 8A is a projection view showing arrangement of marks on the
reticle;
FIG. 8B is an enlarged projection view showing an example of marks
on the reticle;
FIG. 8C is a plan view showing arrangement of the reference marks
on a reference mark plate 6;
FIG. 8D is an enlarged view showing an example of a reference mark
35A or the like;
FIG. 8E is a plan view showing an example of a reference mark 37A
or the like;
FIG. 9 is a plan view for use in describing relation among the
reference mark plate, the reticle, the projection optical system
and the alignment device during measurement of reticle alignment
and a base line amount;
FIG. 10 is a view showing error vectors obtained by means of
measuring the reticle alignment and the base line amount;
FIG. 11 is a partially cutaway structural diagram showing structure
of a reticle alignment microscope 19 and an illumination
system;
FIG. 12A is a view showing an image observed through the image
pick-up device in FIG. 11;
FIGS. 12B and 12C are views showing waveforms indicative of image
signals in an X direction and a Y direction corresponding to the
image shown in FIG. 12A;
FIG. 13 is a structural diagram showing an alignment device 34 of
an off-axis type;
FIG. 14A is a view showing an image observed through the image
pick-up device in FIG. 13;
FIGS. 14B and 14C are views showing waveforms indicative of image
signals in an X direction and a Y direction corresponding to the
image shown in FIG. 14A;
FIGS. 14D and 14E are views showing detection signals obtained
through an LIA optical system shown in FIG. 13;
FIG. 15 is comprised of FIGS. 15A and 15B showing flow charts
illustrating a part of operation of an exposure method and a base
line amount check method according to a second embodiment;
FIG. 16 is a flow chart illustrating a remaining part of the
operation of the alignment method and the base line amount check
method according to the second embodiment; and
FIG. 17 is a flow chart illustrating operation of an exposure
method according to a third embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A projection exposure method according to a first embodiment of the
present invention is now described with reference to the drawings.
This embodiment is when the present invention is applied to a case
where patterns on the reticle are exposed on a wafer by using a
projection type exposure apparatus of a slit scanning exposure
type.
FIG. 1 shows a projection type exposure apparatus according to this
embodiment. In FIG. 1, patterns on a reticle 12 are illuminated
through an illumination area (hereinafter, referred to as a
"slit-shaped illumination area") of a rectangular shape formed by
an exposing light EL from an illuminating optical system which is
not shown in the figure. Images of the patterns are projected and
exposed on a wafer 5 through a projection optical system 8. In this
event, the wafer 5 is scanned backward from the perspective to the
surface of FIG. 1 at a constant velocity V/M (where 1/M is a
reduction magnification of the projection optical system 8) in
synchronism with the scanning of the reticle 12 forward from the
perspective of the surface of FIG. 1 at the constant velocity of V
relative to the slit-shaped illumination area of the exposing light
EL.
Described is a driving system for the reticle 12 and the wafer 5. A
reticle Y-driving stage 10 is mounted on a reticle supporting
platform 9. The reticle Y-driving stage 10 is drivable in a
direction of a Y axis (a direction perpendicular to the paper
surface of FIG. 1). A reticle fine driving stage 11 is mounted on
the reticle Y-direction driving stage 10. The reticle 12 is held by
a vacuum chuck or the like on the reticle fine driving stage 11.
The reticle fine driving stage 11 controls a position of the
reticle 12 slightly with a high accuracy in an X direction parallel
to the paper surface of FIG. 1, Y direction and a rotation
direction (.theta. direction) within a plane orthogonal to the
optical axis of the projection optical system 8. A movable mirror
21 is disposed on the reticle fine driving stage 11. Positions of
the reticle fine driving stage 11 are monitored continuously in the
X, Y and .theta. directions through an interferometer 14 disposed
on the reticle supporting platform 9. Position information S1
obtained by the interferometer 14 is supplied to a main control
system 22A.
A wafer Y-axis driving stage 2 is mounted on a wafer supporting
platform 1. The wafer Y-axis driving stage 2 is drivable in a
direction of the Y-axis. A wafer X-axis driving stage 3 is mounted
on the wafer Y-axis driving stage 2. The wafer X-axis driving stage
3 is drivable in a direction of an X-axis. A Z.theta.-axis driving
stage 4 is so disposed on the wafer X-axis driving state 3 that is
drivable in at least the rotation direction. The wafer 5 is held on
the Z.theta.-axis driving stage 4 by vacuum. A movable mirror 7 is
secured to the Z.theta.-axis driving stage 4. Positions of the
Z.theta.-axis driving stage 4 are monitored in the X, Y and .theta.
directions through an interferometer 13 arranged outside. Position
information obtained by the interferometer 13 is also supplied to
the main control system 22A. The main control system 22A controls
positioning operation, through a wafer driving device 22B or the
like, the wafer Y-axis driving stage 2, the wafer X-axis driving
stage 3 and the Z.theta.-axis driving stage 4 and controls the
operation of the entire device.
As will be described later, a reference mark plate 6 is secured to
the surface of the Z.theta.-axis driving stage 4 at a position
close to the wafer 5 to align a wafer coordinate system and a
reticle coordinate system. The wafer coordinate system is defined
by a coordinate measured by the interferometer 13 at the wafer
side. The reticle coordinate system is defined by a coordinate
measured by the interferometer 14 at the reticle side. Various
reference marks are formed on the reference mark plate 6 as will be
described later. Some of these reference marks (luminous reference
marks) are illuminated from the backside (from the side of the
Z.theta.-axis driving stage 4) by an illumination light led to the
Z.theta.-axis driving stage 4 side.
Reticle alignment microscopes 19 and 20 are provided over the
reticle 12 of this embodiment to observe the reference marks on the
reference mark plate 6 and the marks on the reticle 12
simultaneously. In such a case, deflection mirrors 15 and 16 are
movably disposed to lead the detection light supplied from the
reticle 12 to the reticle alignment microscopes 19 and 20,
respectively. When an alignment sequence begins, the deflection
mirrors 15 and 16 are withdrawn by mirror driving devices 17 and 18
in response to a command supplied from the main control system 22A.
An alignment device 34 of the off-axis type is also provided at a
Y-direction side of the projection optical system 8 to observe the
alignment marks (wafer marks) on the wafer 5.
A keyboard 22C is connected to the main control system 22A,
allowing an operator to enter commands. The projection aligner
according to this embodiment has a quick mode for measuring quickly
the base line amount or the like along with a mode for measuring
the same with a high accuracy as will be described below. An
operator indicates that the mode to be executed is whether the high
accuracy mode or the quick mode through the keyboard 22C to the
main control system 22A.
Next, described with reference to flow charts in FIGS. 2A and 2B is
an operational sequence from loading to the wafer 5 and reticle 12
to completion of the alignment in the projection type exposure
apparatus according to this embodiment. First, at step 101 in FIG.
2A, reticle 12 is subjected to prealignment based on external form
reference on a reticle loader (described below).
FIG. 3 shows a reticle loader system for use in carrying the
reticle 12 to the reticle fine driving stage 11 shown in FIG. 1.
The reticle loader in FIG. 3 comprises two reticle arms 23A and
23B, an arm rotation axis 25 connected to the reticle arms 23A and
23B, and a rotation mechanism 26 for rotating the arm rotation axis
25. Grooves 24A and 24B for vacuuming are formed in a reticle
mounting surface of the reticle arms 23A and 23B, respectively. The
reticle arms 23A and 23B are so supported that they can rotate
independently of each other through the arm rotation axis 25.
During loading of the reticle 12, the reticle 12 is passed to the
reticle arm 23A through a reticle carrying mechanism (not shown) at
a position A3. At that time, the other reticle arm 23B is used for
carrying, for example, another reticle used during a previous
process. A reticle configuration prealignment mechanism (not shown)
mounted near the position A3 aligns the reticle 12 on the reticle
arm 23A based on the external form thereof with a predetermined
accuracy. Subsequently, the reticle 12 is subjected to vacuum
suction to the reticle arm 23A. Next, at step 102 in FIG. 2A, the
rotation mechanism 26 rotates the reticle arm 23A through the arm
rotation axis 25 to move the reticle 12 to a position B3 in the Y
direction (a ready position (passing position) of the reticle
driving stage 10 in FIG. 1).
In this event, the groove 24A for vacuum suction is a groove
extending in a direction orthogonal to a suction position (parallel
direction of the groove 24A) on the reticle fine driving stage 11
and is located out of a pattern area of the reticle 12.
Accordingly, the reticle arm 23A is allowed to advance and retract
the reticle 12 freely to and from the reticle fine driving stage 11
with the reticle fine driving stage 11 moved to the topmost in the
y direction or the scanning direction. When the reticle 12 arrives
over the reticle fine driving stage 11 (FIG. 1), the arm rotation
axis 25 retracts in a -Z direction. The reticle 12 is thus mounted
on the vacuum suction surface of the reticle fine driving stage 11.
After completion of passing of the reticle 12, the reticle arm 23A
is withdrawn. Subsequently, the reticle fine driving stage 11
carries the reticle 12 towards a position C3. In this event, the
reticle arms 23A and 23B are driven independently of each other.
The reticle passing speed is increased by means of simultaneously
performing loading and unloading of the reticles by these arms.
Next, at steps following step 103 the reticle 12 is aligned. A
mechanism and operation therefor are described.
FIG. 4A shows arrangement of alignment marks (reticle marks) on the
reticle 12. FIG. 4B shows a slit-shaped illumination area 32 or the
like within an area 33R conjugated with an effective exposing field
of the projection optical system over the reticle. A y direction
corresponds to a scanning direction while an x direction
corresponds to a direction orthogonal to the y direction. In FIG.
4A, a shield portion 31 is formed along a periphery of a pattern
area at the center of the reticle 12. Reticle marks formed outside
the shield portion 31 are: rough searching alignment marks 27 and
28 and fine alignment marks 29A, 29B, 29C, 29D, 30A, 30B, 30C and
30D. The rough searching alignment mark 27 at the right-hand side
is formed of an elongated linear pattern and cross patterns. The
linear pattern extends along the y direction or the scanning
direction and the cross patterns are formed at both ends of the
linear pattern. The searching alignment mark 28 at the left-hand
side is formed symmetrically with the rough searching alignment
mark 27 at the right-hand side.
The fine alignment marks 29A and 29B are provided near the y
direction between the shield portion 31 at the right-hand side and
one cross pattern of the rough searching alignment mark 27. The
fine alignment marks 29C and 29D are provided near the y direction
between the shield portion 31 at the right-hand side and the other
cross pattern of the rough searching alignment mark 27. The fine
alignment marks 30A, 30B, 30C and 30D are formed at the left-hand
side symmetrically with the fine alignment marks 29A, 29B, 29C and
29D, respectively. While each of the fine alignment marks 29A, 29B,
29C, 29D, 30A, 30B, 30C and 30D is shown as a cross mark in FIG.
4A, it is formed of two sets of three linear patterns arranged in
the x direction at a predetermined distance and two sets of three
linear patterns arranged in the y direction at a predetermined
distance as shown in FIG. 4C. At step 103 in FIG. 2A, the rough
searching alignment mark 28 at the left-hand side in FIG. 4A is
detected by the reticle alignment microscope (hereinafter referred
to as "RA microscope") 20. FIG. 4B shows observation areas 19R and
20R of the RA microscopes 19 and 20, respectively, on the reticle
12. During rough searching, the rough searching alignment marks 27
and 28 are located out of the observation areas 19R and 20R and
also out of an area 33R conjugated with the effective exposing
field. This is because the rough searching alignment marks 27 and
28 should be large enough for rough searching but the exposing
field of the projection optical system is not so large, otherwise
the diameter of a projection lens should be increased, resulting in
increase of the costs. With this respect, procedures for performing
the rough search in this embodiment is described in conjunction
with FIGS. 5A and 5B.
FIG. 5A is an enlarged view of one cross pattern and its periphery
of the rough searching alignment mark 28. FIG. 5B is a reduced
version of FIG. 5A. In FIGS. 5A and 5B, W represents a width of a
square effective field of view 20Ref of the RA microscope 20 and
.DELTA.R represents a designed value of a sum of a writing error
and a mounting error of the patterns relative to the outer
configuration of the reticle 12. Accordingly, as shown in FIG. 5B,
one cross pattern 28a of the rough searching alignment mark 28 is
always contained within a square area having the width of .DELTA.R.
What are to be detected are x and y coordinates of the cross
pattern 28a. In this embodiment, the effective field of view
20R.sub.ef having the width of W is scanned diagonally to the x and
y axes in a direction passing at 45.degree. to two axes of the
alignment mark 28. The x and y coordinates of the cross pattern 28a
are obtained as the x and y coordinates at that time when the
alignment mark 28 is diagonally scanned.
For this purpose, an integer portion of a positive real number is
represented by INTA. The number of search fields or the least
number of scanning of the square area having the width .DELTA.R
with the effective field of view 20R.sub.ef having the width W can
be given by {INTA (.DELTA.R/W)+1}. This number of search fields is
previously obtained. The {INTA (.DELTA.R/W)+1} effective fields of
view A5, B5, C5, . . . , each having the width W are set diagonally
to the square area having the width .DELTA.R around the first
effective field of view B5. The reticle fine driving stage 11 shown
in FIG. 1 is driven to step the effective fields of view, thereby
sampling images within each effective field of view while setting
them successively within the effective field of view 20R.sub.ef in
FIG. 5A.
As shown in FIG. 5B, the cross pattern 28a of the alignment mark 28
to be searched is present within a search boundary having the width
and length of at least .DELTA.R.times..DELTA.R. The alignment mark
28 is large enough to the search boundary. Accordingly, to step the
effective fields of view in a diagonal direction to the alignment
mark 28 makes it possible to detect the coordinates of the cross
pattern 28a of the alignment mark 28 with a least number of fields.
The image processing at that time may be a one-dimensional image
processing on image signals obtained by means of adding the
scanning lines of all lines within the picked-up image.
FIGS. 6A to 6F show image signals obtained by means of so adding
the scanning lines of all lines. FIGS. 6A and 6D represent image
signals obtained along the x and y directions, respectively, within
the effective field of view A5 in FIG. 5B. FIGS. 6B and 6E
represent image signals obtained along the x and y directions,
respectively, within the effective field of view B5 in FIG. 5B.
FIGS. 6C and 6F represent image signals obtained along the x and y
directions, respectively, within the effective field of view C5 in
FIG. 5B. The x coordinate of the cross pattern 28a is obtained from
the image signal shown in FIG. 6B while the y coordinate thereof is
obtained from the image signal shown in FIG. 6F.
After detecting the searching reticle mark 28 in this manner, the
rough searching alignment mark 27 is moved to the observation area
of the RA microscope 19 at step 104 in FIG. 2A. The position of the
alignment mark 27 is detected in the same manner as described
above. In this event, the portion of the reference mark plate 6
where no pattern is included is moved within the exposing field of
the projection optical system 8 to illuminate the portion from the
bottom. The illumination light emitted from the reference mark
plate 6 allows illumination of the rough searching alignment marks
27 and 28 from the backside (z.theta.-axis driving stage side).
The above mentioned sequence roughly aligns the position of the
rough searching alignment marks 27 and 28 and the reticle
coordinate system relative to the observation areas 19R and 20R of
the RA microscopes 19 and 20 in FIG. 4B. In addition, rough
alignment of the observation areas 19R and 20R of the RA
microscopes with the wafer coordinate system can be made by means
of measuring the reference marks on the reference mark plate 6 in
FIG. 1 through the RA microscopes 19 and 20. As a result, rough
alignment is completed such that the fine alignment marks 29A, 29B,
29C, 29D, 30A, 30B, 30C and 30D are not overlapped with the
reference marks (described below) of the reference mark plate
6.
In this embodiment, the alignment marks on the reticle 12 are
formed of the rough searching alignment marks and the fine
alignment marks for the purpose of reducing the diameter of the
lens of the projection optical system 8. However, the rough
searching alignment marks may be used as the fine alignment marks
when a lens of the larger diameter can be available. In such a
case, searching can be made in the same manner on the alignment
marks by means of stepping in a diagonal direction as shown in
FIGS. 5A and 5B. The searching of the alignment marks can be made
simultaneously through the RA microscopes 19 and 20.
Next, a sequence for fine alignment is described. Detailed
structure of the wafer stage and the reticle stage is described
first.
FIG. 7A is a plan view of the wafer stage. In FIG. 7A, the wafer 5
and the reference mark plate 6 are disposed on the Z.theta.-axis
driving stage 4. Movable mirrors 7X and 7Y for the X and Y axes,
respectively, are secured to the Z.theta.-axis driving stage 4. A
slit-shaped illumination area 32W, corresponding to the slit-shaped
illumination area 32 in FIG. 4B, is illuminated by an exposing
light on the wafer 5. Observation areas 19W and 20W are conjugated
with the observation area 19R and 20R, respectively, in FIG.
4B.
Laser beams LWX and LW.sub.of are directed to the movable mirror 7X
at a distance IL in the direction parallel to the X axis along
optical paths passing the optical axis of the projection optical
system and a reference point of the alignment device 34,
respectively. Laser beams LWY1 and LWY2 are directed to the movable
mirror 7Y along optical paths parallel to the Y axis. During
alignment and exposure, a coordinate value measured by an
interferometer using the laser beam LWX is used as the X coordinate
of the Z.theta.-axis driving stage 4. Used as the Y coordinate is
an average (Y.sub.1 +Y.sub.2)/2 of coordinate values Y.sub.1 and Y2
measured by interferometers using the laser beams LWY1 and LWY2,
respectively. For example, the rotation amount in the rotation
direction (.theta. direction) of the Z.theta.-axis driving stage 4
can be measured according to the difference between the coordinate
values Y.sub.1 and Y.sub.2. The position and a rotation amount
within an XY plane of the Z.theta.-axis driving stage 4 is
controlled according to these coordinates.
In particular, for the Y direction or the scanning direction, an
error due to air fluctuation or the like during scanning is
relieved using an averaging effect by means of applying the average
value of the measured results obtained by two interferometers. When
the alignment device 34 of the off-axis type is used, the position
in the X-axis direction is so controlled as not to cause a
so-called Abbe's error according to measured values of an exclusive
interferometer using the laser beam LW.sub.of.
FIG. 7B is a plan view of the reticle stage. In FIG. 7B, the
reticle fine driving stage 11 is mounted on the reticle Y-axis
driving stage 10, on which the reticle 12 is held. A movable mirror
21x for the x axis and two movable mirrors 21y1 and 21y2 for the y
axis are secured to the reticle fine driving stage 11. A laser beam
LRx is directed to the movable mirror 21x in parallel with the x
axis. Laser beams LRy1 and LRy2 are directed to the movable mirrors
21y and 21y2, respectively, in parallel with the y axis.
As in the case of the wafer stage, a coordinate in the y direction
of the reticle fine driving stage 11 is an average value of
(y.sub.1 +y.sub.2)/2 of coordinates y.sub.1 and y.sub.2 measured by
two interferometers using the laser beams LRy1 and LRy2,
respectively. A coordinate in the x direction is a coordinate value
measured by an interferometer using the laser beam LRx. The
rotation amount in a direction (.theta. direction) of the reticle
fine driving stage 11 is measured according to the difference
between, for example, the coordinate values y.sub.1 and
y.sub.2.
In this event, corner-cube reflector elements are used as the
movable mirrors 21y1 and 21y2 in the y direction or the scanning
direction. The laser beams LRy1 and LRy2 reflected from the movable
mirrors 21y1 and 21y2 are in turn reflected back from reflection
mirrors 39 and 38, respectively. More specifically, the
interferometer for the reticle is a double-path interferometer.
Accordingly, rotation of the reticle fine driving stage 11 does not
shift or displace the position of the laser beams. As in the case
of the wafer stage, the reticle 12 is provided with the slit-shaped
illumination area 32 and the observation areas 19R and 20R of the
RA microscopes 19 and 20, respectively. The Z.theta.-axis driving
stage 4 in FIG. 7A and the reticle 12 can be observed only through
the observation areas 19R and 20R. A relation between the reticle
12 and the Z.theta.-axis driving stage 4 is so measured as to
improve the rotation accuracy of the reticle 12 and the wafer 5 as
well as the alignment accuracy on exposing. A method thereof is
described in conjunction with FIGS. 8A, 8B, 8C, 8D, 8E and 9.
FIG. 8A shows an reticle image 12W obtained by means of projecting
the reticle 12 to the reference mark plate 6 in FIG. 7A. In FIG.
8A, shown are mark images 29AW, 29BW, 29CW and 29DW conjugated with
the fine alignment marks 29A, 29B, 29C and 29D, respectively, in
FIG. 4A and mark images 30AW, 30BW, 30CW and 30DW conjugated with
the fine alignment marks 30A, 30B, 30C and 30D, respectively. Each
of the mark images 29AW, 29BW, 29CW, 29DW, 30AW, 30BW, 30CW and
30DW is formed of four sides each comprising three linear patterns
as shown in FIG. 8B.
FIG. 8C shows arrangement of reference marks on the reference mark
plate 6. Formed on the reference mark plate 6 in FIG. 8C are
reference marks 35A, 35B, 35C, 35D, 36A, 36B, 36C and 36D arranged
in a manner similar to the mark images 29AW, 29BW, 29CW, 29DW,
30AW, 30BW, 30CW and 30DW in FIG. 8A. These reference marks are
illuminated by an illumination light that is equal in wavelength to
the exposing light. A reference mark 37A is also provided on the
reference mark plate 6 at a position away from a center between the
reference marks 35A and 36A at a distance IL in the Y direction or
the scanning direction. The distance IL corresponds to the base
line amount, the distance between the reference point of the
projection optical system 8 in FIG. 1 and the reference point of
the alignment device 34 of the off-axis type. Likewise, reference
marks 37B, 37C and 37D are formed as positions away from centers
between the reference marks 35B and 36B, between the reference
marks 35C and 36C, and between the reference marks 35D and 36D,
respectively, at a distance IL in the Y direction.
Each of the reference marks 35A, 35B, 35C, 35D, 36A, 36B, 36C and
36D is formed of linear patterns of 7-row by 7-column as shown in
FIG. 8D. The reference marks 35A, 35B, 35C, 35D, 36A, 36B, 36C, and
36D have sizes smaller than the mark images 29AW, 29BW, 29CW, 29DW,
30AW, 30BW, 30CW and 30DW in FIG. 8B. The reference marks 37A, 37B,
37C and 37D are, as shown in FIG. 8E, associated lattice points of
a grid pattern formed at a predetermined pitch in the X and Y
directions.
In such a case, at step 105 in FIG. 2A, a relative position
relation and a relative rotation angle of the reticle 12 and the RA
microscopes 19 and 20 are calculated on the basis of the results
obtained at the steps 103 and 104 to move the fine alignment marks
29A and 30A in FIG. 4A into the-observation area 19R and 20R of the
RA microscopes 19 and 20, respectively. Subsequently, at step 106,
the reference marks 35A and 36A on the reference mark plate 6 in
FIG. 8C are moved into the observation areas 19W and 20W (see FIG.
9) conjugated with the observation areas 19R and 20R, respectively.
As a result, the mark image 29AW and the reference mark 35A are
observed simultaneously within the observation area 19W and the
mark image 30AW and the reference mark 36A are observed
simultaneously within the observation area 20W as shown in a
portion depicted by 220 in FIG. 9. Subsequently, at step 107 in
FIG. 2A, the images observed through the RA microscopes 19 and 20
are converted into image pick-up signals and sampled. At the same
time, the detection signals of the associated reference mark images
are also sampled in the alignment device 34 of the off-axis
type.
At the portion 220 in FIG. 9, the reticle image 12W or the
projection image of the reticle is projected to the reference mark
plate 6. As shown in a portion 222 in FIG. 9, the observation areas
19W and 20W are located at positions passing the optical axis
within the exposing field of the projection optical system 8. The
reference mark 37A is within the observation area of the alignment
device 34 of the off-axis type. As in the case of the slit scanning
exposure, the reticle fine driving stage 11 in FIG. 7B is moved
downward (to a -y direction) in synchronism with movement of the
Z.theta.-axis driving stage 4 in FIG. 7A upward (to the Y
direction). As a result, the reference mark plate 6 and the reticle
image 12W are moved together to the Y direction as shown in 220 and
221 in FIG. 9. In this event, the observation areas 19W and 20W of
the RA microscopes 19 and 20 and the alignment device 34 of the
off-axis type are all fixed, so that from a set of marks with a
symbol A (the mark images 29AW, 30AW, the reference marks 35A, 36A
and 37A) to a set of marks with a symbol D (the mark images 29DW,
30DW, the reference marks 35D, 36D and 37D) are moved under the
observation areas 19W and 20W and the alignment device 34.
At a first stop position after initiation of alignment shown in the
portion 220 in FIG. 9, the mark image 29AW and the reference mark
35A are located under the observation area 19W. The mark image 30A
and the reference mark 36A are located under the observation area
20W. The reference mark 37A is located under the alignment device
34 of the off-axis type. These marks with the symbol A are all
observed at the same time. After completion of measurement at the
first stop position, the reticle image 12W and the reference mark
plate 6 are moved synchronously to a second stop position by the
stepping operation. The set of marks observed at the first stop
position under the observation areas 19W and 20W and the alignment
device 34 is a set of marks with the symbol A while the set of
marks present at this second stop position under the observation
areas 19W and 20W and the alignment device 34 is a set of marks
with a symbol B (the mark image 29BW in FIG. 8A, the reference
marks 35B, 37B in FIG. 8C or the like).
By means of repeating the above mentioned sequence for third and
fourth stop positions (as shown in 221 in FIG. 9), the mark image
of the reticle image 12W and the reference marks on the reference
mark plate 6 are measured through the RA microscopes 19 and 20 and
the alignment device 34 of the off-axis type in the order of the
set of marks with the symbol A, the set of marks with the symbol B,
the set of marks with the symbol C and the set of marks with the
symbol D. This corresponds-to the operation illustrated in the
steps 105 through 110 in FIGS. 2A and 2B. The so obtained measured
result is clearly shown in FIG. 10.
In FIG. 10, a vector of the alignment error of the reference mark
35A through the mark image 29AW is referred to as AL that is
obtained by means of correcting the measured result obtained
through the RA microscope 19 in the following manner. Likewise,
vectors of the alignment errors of the reference marks 35B, 35C and
35D through the mark image 29BW, 29CW and 29DW are referred to as
BL, CL and DL, respectively. Likewise, .DELTA.R represents a vector
of the alignment error of the reference mark 36A through the mark
image 30AW. BR, CR and DR represent vectors of the alignment errors
of the reference marks 36B, 36C and 36D through the mark image
30BW, 30CW and 30DW, respectively. In addition, an error vector
from the reference marks 37A, 37B, 37C and 37D to the reference
point of the alignment device 34 are referred to as AO, BO, CO and
DO, respectively, that are obtained by means of correcting the
measured result obtained through the alignment device 34 of the
off-axis type in a manner described below.
ReAx, ReBx, ReCx and ReDx represent the coordinate values in the x
direction measured by the interferometer 14 at the reticle side in
FIG. 1, i.e., the coordinate values obtained by using the laser
beam LRx in FIG. 7B when the error vectors AL, AR, BL, BR, CL, CR,
DL and DR are obtained. ReAy1, ReBy1, ReCy1, ReDy1, ReAy2, ReBy2,
ReCy2 and ReDy2 represent the coordinate values in the y direction
measured by the interferometer 14 at the reticle side in FIG. 1,
i.e., the coordinate values obtained by using the laser beams LRy1
and LRy2 in FIG. 7B when the error vectors AL, AR, BL, BR, CL, CR,
DL and DR are obtained. WeAx, WaBx, WaCx and WaDx represent the
coordinate values in the X direction measured by the interferometer
13 at the wafer side in FIG. 1, i.e., the coordinate values
obtained by using the laser beam LWX in FIG. 7A when the error
vectors AL, AR, BL, BR, CL, CR, DL and DR are obtained. WaAY1,
WaBY1, WaCY1, WaDY1, WaAY2, WaBY2, WaCY2 and WaDY2 represent the
coordinate values in the Y direction measured by the interferometer
13 at the reticle side in FIG. 1, i.e., the coordinate values
obtained by using the laser beams LWY1 and LWY2 in FIG. 7A when the
error vectors AL, AR, BL, BR, CL, CR, DL and DR are obtained.
WaAOX, WaBOX, WaCOX and WaDOX represent the coordinate values in
the X direction measured by the interferometer exclusive for the
alignment device of the off-axis type at the wafer side in FIG. 1,
i.e., the coordinate values obtained by using the laser beam
LW.sub.OF in FIG. 7A when the error vectors AO, BO, CO and DO. In
this event, as shown in FIG. 7A, the distance between the laser
beams LWY1 and LWY2 at the wafer side in the X direction is IL and
the distance at the wafer side between the laser beams LRy1 and
LRy2 at the reticle side is RL.
Next, structure of the RA microscope 19 in FIG. 1 is described in
detail for use in describing how to obtain the error vector AL or
the like.
FIG. 11 shows the RA microscope 19 and its illumination system. In
FIG. 11, an illumination light EL having the same wavelength as the
exposing light is introduced into the Z.theta.-axis driving stage 4
from the outside of the Z.theta.-axis driving stage 4 through an
optical fiber 44. An exposing light may be relayed through lens
systems rather than using the optical fiber 44. The so introduced
illumination light illuminates the reference marks 35A, 35B, 35C
and 35D on the reference mark plate 6 through a lens 45A, a beam
splitter 45B and a lens 45C. The illumination light transmitted
through the beam splitter 45B illuminates the reference marks 36A,
36B, 36C and 36D on the reference mark plate 6 through a lens 45D,
a lens 45E, a mirror 45F and a lens 45G.
For example, the light transmitted through the reference mark 35A
focuses an image of the reference mark 35A on the file alignment
mark 29A on the reticle 12. The light from the image of the
reference mark 35A and the alignment mark 29A is reached to a half
mirror 42 through a deflection mirror 15, a lens 40A and a lens
40B. The light divided into two portions through the half mirror 42
are directed to image pick-up surfaces of image pick-up devices 43X
and 43Y, respectively, for the X and Y axes, each of which being
formed of two dimensional charged-coupled-device (CCD). The image
35AR of the reference mark 35A and the fine alignment mark 29A as
shown in FIG. 12A are projected on the image pick-up devices 43Y
and 43X, respectively. In this event, an image pick-up field 43Xa
of the image pick-up device 43 for the X axis is an area parallel
to the X direction of the wafer stage and the direction of the
horizontal scanning lines also corresponds to the X direction. An
image pick-up field 43Ya of the image pick-up device 43Y for the Y
axis is an area parallel to the Y direction of the wafer stage and
the direction of the horizontal scanning line corresponds to the Y
direction.
Accordingly, a displacement amount in the x direction between the
reference mark 35A and the alignment mark 29A can be obtained
according to an averaging of image pick-up signals S4X obtained by
the image pick-up device 43X. A displacement amount in the Y
direction between the reference mark 35A and the alignment mark 29A
can be obtained according to an averaging of image pick-up signals
S4Y obtained by the image pick-up device 43Y. These image pick-up
signals S4X and S4Y are supplied to a signal processing device 41
(see FIG. 11).
More specifically, description is made in conjunction with an
exemplified case where the set of marks with the symbol A is
subjected-to alignment. For example, the alignment mark 29A and the
reference mark 35AR shown in FIG. 12A are observed through the RA
microscope 19 at the same time. In FIG. 12A, the image signals S4X
and S4Y within the image pick-up fields 43Xa and 43Ya enclosed by
broken lines are detected as digital signals by means of carrying
out an analog-to-digital conversion in the signal processing device
41. The image data on the individual scanning lines are averaged
independently for the X and Y axes in the signal processing device
41. The averaged image signals S4X' and S4Y' for the X and Y axes,
respectively, are as shown in FIGS. 12B and 12C, respectively.
These image data are processed as one-dimensional image processing
signals.
Calculation and processing on the so obtained signals in the signal
processing device 41 produces relative displacements AL'.sub.X and
AL'.sub.Y in the X and Y directions between the mark image 29AW of
the reticle 12 and the reference mark 35A of the reference mark
plate 6 in FIG. 10. Using the RA microscope 20 in FIG. 1, obtained
are relative displacements AR'.sub.X and AR'.sub.Y in the X and Y
directions between the mark image 30AW and the reference mark 36A.
Likewise, obtained are relative displacements between the mark
images 29BW, 29CW and 29DW and the reference marks 35B, 35C and
35D, respectively, in FIG. 10 and relative displacements between
the mark images 30BW, 30CW and 30DW and the reference marks 36B,
36C and 36D, respectively.
However, the image signal corresponding to the alignment mark 29A
and the image signal corresponding to the reference mark image 35AR
in FIG. 12B are controlled in position by the interferometer at the
reticle side and the interferometer at the wafer side,
respectively. Accordingly, measurement errors (=measured value-set
value), .DELTA.ReAx, .DELTA.ReAy1, .DELTA.ReAy2, .DELTA.WaAX,
.DELTA.WaAY1, and .DELTA.WaAY2 are caused due to following errors
at the individual stages with respect to the measured coordinates
ReAx, ReAy1 and ReAy2 obtained by the interferometer at the reticle
side and the measured coordinates WaAX, WaAY1 and WaAY2 obtained by
the interferometer at the wafer side during measurement of the set
of marks with the symbol A (29AW, 35A, 30AW and 36A in FIG. 10).
These measurement errors are contained in the relative
displacements AL'.sub.X and AL'.sub.Y obtained through the
calculation.
With this respect, the results obtained by means of subtracting the
errors from the relative displacements obtained through measurement
correspond to an X component AL.sub.X and a Y component AL.sub.Y of
the vector of the alignment error in FIG. 10. In this event, (1/M)
in a following equation represents a reduction magnification of the
projection optical system 8, while IL and RL represent the
distances described in conjunction with FIG. 7A.
and ##EQU1##
Likewise, an X component AR.sub.X and a component AR.sub.Y of the
vector AR of the alignment error in FIG. 10 can be given by
following equations.
and ##EQU2##
Next, structure of the alignment device 34 is described in
conjunction with FIG. 13 for use in describing the error vectors
AO, BO, CO and DO in FIG. 10 obtained by means-of correcting the
results obtained by the alignment device 34 of the off-axis
type.
FIG. 13 shows structure of the alignment device 34. In FIG. 13, the
light from the reference mark on the reference mark plate 6 is
deflected from a deflection mirror unit 46 and is directed to a
half prism 47. The light reflected from the half prism 47 is
directed to an alignment optical system 48 (hereinafter, referred
to as "FIA optical system") of an image processing type using white
light. The light transmitted through the half mirror is directed to
an alignment optical system 52 (hereinafter, referred to as "LIA
optical system") for use in detecting the diffraction light from
lattice marks using a heterodyne beam.
Describing about the FIA optical system 48, the illumination light
from an illumination light source 49 is passed through the FIA
optical system 48 and is deflected through the half prism 47 and
the deflection mirror 46 to illuminate the reference marks on the
reference mark plate 6. The back light therefrom goes back to the
FIA optical system through the same optical path. The light
transmitted through the FIA optical system is directed to a half
prism 50A. The light beam transmitted through the half prism 50A
focuses an image of the reference mark of the reference mark plate
6 on the image pick-up surface of an image pick-up device 51X for
the X axis formed of a two-dimensional CCD. The light beam
reflected from the half prism 50A focuses an image of the reference
mark of the reference mark plate 6 on the image pick-up surface of
an image pick-up device 51Y for the Y axis formed of a
two-dimensional CCD.
On the image pick-up surfaces of the image pick-up devices 51X and
51Y, images shown in FIG. 14A are focused. The reference marks on
the reference mark plate 6 are grating points of the grating
pattern. FIG. 14A shows an image 37P of the grating pattern. It is
assumed that P and L represent grating pitch on the reference mark
plate 6 of the image 37P of the grating pattern and a width of a
dark line, respectively, the width L is significantly smaller than
the pitch P. Focused on the image pick-up surface are reference
mark (index mark) images 48X1 and 48X2 in the X direction and index
mark images 48Y1 and 48Y2 in the Y direction illuminated by another
illumination light other than the illumination light for the
reference mark plate 6. The position of the reference marks on the
reference mark plate 6 can be detected with the position of the
index marks as the reference.
More specifically, image pick-up area 51Xa and 51Ya in the
directions conjugated with the X and Y directions, respectively, in
FIG. 14A are picked up through the image pick-up devices 51X and
51Y in FIG. 13. The directions of the horizontal scanning lines of
the image pick-up devices 51X and 51Y are directions conjugated
with the X and Y directions, respectively. Image pick-up signals
S5X and S5Y obtained by the image pick-up devices 51X and 51Y,
respectively, are supplied to a signal processing device 56 in FIG.
13. The signal processing device 56 averages the image pick-up
signals S5X and S5Y to produce image signals S5X' and S5Y' shown in
FIGS. 14B and 14C, respectively. A displacement of the directed
reference mark on the reference mark plate 6 is obtained according
to these image signals. More detailed structure is disclosed in
Japanese Patent Application No. 4-16589.
It is assumed that relative displacements in the X and Y directions
of the reference mark 37A relative to the reference mark obtained
as a result of the image processing in FIG. 14A are represented by
AO'.sub.fX and AO'.sub.fY when the reference mark to be detected is
the reference mark 37A in FIG. 10. In this event, the position of
the reference mark plate 6 is controlled on the wafer coordinate
system, so that values obtained by subtracting the follow error and
the rotation error of the Z.theta.-axis driving stage 4 in FIG. 7A
from the measured result are an X component AO.sub.X and a Y
component AO.sub.Y of the error vector AO in FIG. 10. The X
component AO.sub.X and the Y component AO.sub.Y corresponding to
the FIA optical system 48 in FIG. 13 are represented by AOf.sub.X
and AO.sub.fY. That is, following equations are given:
and
On the other hand, in the alignment system containing the LIA
optical system 52 in FIG. 13, a laser beam emitted from a laser
beam source 53 is transmitted through the LIA optical system 52 and
the half prism 47, which is then deflected from the deflection
mirror 46 and directed to the reference mark of a diffraction
grating shape on the reference mark plate 6. The diffracted light
from the reference mark goes back to the LIA optical system 52
through the same optical path. The diffracted light transmitted
through the LIA optical system 52 is divided into two portions
through a half prism 50B and directed to photosensitive elements
55X and 55Y for the X and Y directions, respectively.
In this event, the laser beam emitted from the laser beam source 53
in the LIA optical system 52 is divided into two portions. A
frequency difference of .DELTA.f is caused between frequencies of
these two laser beams by an internal frequency shifter.
Interference light of these two laser beams is received by a
photosensitive element 54. The photosensitive element 54 produces a
reference signal S6 having a frequency of .DELTA.f. Two laser beams
having different frequencies (heterodyne beams) are directed to the
reference mark of the diffraction grating shape on the reference
mark plate 6 at an adequate incident angle. A .+-.1 order
diffracted light of these two laser beams from the reference mark
returns in parallel in an orthogonal manner relative to the
reference mark plate 6. An interference light of the .+-.1 order
light has an intensity varied at the frequency .DELTA.f and a phase
thereof varies depending on the X and Y coordinates of the
reference mark. The photosensitive element 55X produces a beat
signal S7X having the frequency .DELTA.f and a phase varied
depending on the X coordinate of the reference mark. The
photosensitive element 55Y produces a beat signal S7Y having the
frequency .DELTA.f and a phase varied depending on the Y coordinate
of the reference mark. The reference signal S6 and the beat signals
S7X and S7Y are supplied to the signal processing device 56.
The signal processing device 56 in FIG. 13 calculates a
displacement AO'Lx in the X direction of the reference mark 37A
according to a phase difference .DELTA..phi..sub.X between the
reference signal S6 and the beat signal S7X as shown in FIG. 14D
and calculates a displacement AO'.sub.LX in the Y direction of the
reference mark 37A according to a phase difference
.DELTA..phi..sub.Y between the reference signal S6 and the beat
signal S7X as shown in FIG. 14E when the reference mark to be
detected is the reference mark 37A in FIG. 10. When the following
error and the rotation error of the Z.theta.-axis driving stage 4
in FIG. 7A are subtracted from the above calculation result, the X
component AO.sub.X and Y component AO.sub.Y of the error vector in
FIG. 10 can be obtained. Let the X and Y components AO.sub.X and
AO.sub.Y of the LIA optical system 52 in FIG. 13 be AO.sub.LX and
AO.sub.LY, respectively. That is, the following equation can be
given:
and
In the above mentioned manner, eight data AL.sub.X, AL.sub.Y,
AR.sub.X, AR.sub.Y, AO.sub.fX, AO.sub.fY, AO.sub.LX and AO.sub.LY
are measured by means of performing alignment at the positions of
the mark group with the symbol A in FIG. 10. Measurement on the
mark groups with the symbols A through D in such a sequence
produces thirty-two (=8.times.4) data. Of these thirty-two data,
the data obtained through the RA microscopes 19 and 20 are stored
as measured data D.sub.xn, D.sub.yn while the data obtained through
the alignment device 34 of the off-axis type is stored as measured
data A.sub.xn, A.sub.yn. Subsequently, the operation proceeds to
step 111 in FIG. 2B. At the step 111 in FIG. 2B, it is assumed that
coordinates in the x and y directions are F.sub.xn and F.sub.yn on
a coordinate system where the reticle coordinate system and the
wafer coordinate system are adapted to convert with only the linear
error with respect to the measured data D.sub.xn, D.sub.yn
corresponding to the RA microscopes 19 and 20. The following
relation holds: ##EQU3##
In addition, let nonlinear errors in the x and y directions be
.epsilon..sub.xn and .epsilon..sub.yn, respectively, then the
following equation can be given: ##EQU4##
Six parameters in Equation 9, Rx, Ry, .theta., .omega., Ox and Oy,
are calculated by using the least square approximation to minimize
the nonlinear error (.epsilon..sub.xn, .epsilon..sub.yn). In this
event, the scaling parameter Rx in the x direction indicates a
magnification error in the x direction between the reticle 12 and
the reference mark plate 6. The scaling parameter Ry indicates a
scaling error in the scanning direction (y direction) between the
reticle coordinate system and the wafer coordinate system. The
angle parameter .theta. represents the rotation error between the
reticle 12 and the reference mark plate 6. The angle parameter
.omega. represents the parallelism in the scanning direction of the
reticle coordinate system and the wafer coordinate system. The
offset parameters Ox and Oy represent offset values in the x and y
directions, respectively.
Next, at steps 112 and 113 in FIG. 2B, the base line amount is
obtained. Let averages of the data A.sub.xn and A.sub.yn measured
through the alignment device 34 of the off-axis type be <Ax>
and <Ay>, respectively, then the offset during measurement of
the base line amount becomes (<Ax>-Ox, <Ay>-Oy).
Accordingly, control should be switched during alignment from the
interferometer using the laser beam LWX in FIG. 7A (hereinafter,
also refereed to as "exposing interferometer LWX") to the
interferometer using the laser beam LW.sub.OF (hereinafter, also
referred to as "off-axis exclusive interferometer LW.sub.Of ").
When the FIA optical system 48 in FIG. 13 is used, averages of the
measured data A.sub.xn and A.sub.yn are represented by
<Afx>and <Afy>, respectively. Then, the offset of the
offset (<Afx>-Ox, <Afy>-Oy) is taken into consideration
in the measured values of the interferometer corresponding to the
laser beams LWY1, LWY2 and LW.sub.OF to perform the alignment. On
the other hand, when the LIA optical system 52 in FIG. 13 is used,
averages of the measured data A.sub.xn and A.sub.yn are represented
by <ALx> and <ALy>, respectively. Then, the offset
(<ALx>-Ox, <ALy>-Oy) is taken into consideration in the
measured values of the interferometer.
According to the above, the base line amount obtained corresponds
to what the base line amount (distance IL) is corrected with the
offset of (<Afx>-Ox, <Afy>-Oy) or the offset of
(<ALx>-Ox, <ALy>-Oy).
The above mentioned correction technique means that the reference
coordinate system of the stage coordinate system is set according
to the reference marks on the reference mark plate 6. In such a
case, in other words, an axis passing the reference marks 37A, 37B,
37C and 37D on the reference mark plate 6 serves as a reference
axis, and obtained is a read value (yawing value) of the off-axis
exclusive interferometer LW.sub.OF on this reference axis with the
read value of the exposing interferometer LWX of zero on this
reference axis. During exposure, by using, as "values of the
interferometers for delivery", the read value of the exposing
interferometer LWX and the result of yawing value correction made
on the read value (yawing value) of the off-axis exclusive
interferometer LW.sub.OF, the positioning of the wafer 5 is
performed according to these values for the delivery.
On the other hand, in FIG. 7A for example, an alternative method
may be used where the movable mirror 7X for the X axis is used as
the reference axis of the stage coordinate system. In such a case,
in a condition shown in FIG. 7A, the read values of the exposing
interferometer LWX and the off-axis exclusive interferometer LWoF
are reset simultaneously (to zero), the measured values are used
for the subsequent exposure without using the interferometer values
for delivery. On the other hand, during alignment, obtained is an
inclined angle .theta..sub.XF of the reference axis passing the
reference marks 37A, 37B, 37C and 37D on the reference mark plate 6
relative to the movable mirror 7X. Then, such a value is used that
is obtained by means of correcting with IL.multidot..theta..sub.XF
on the read value of the off-axis exclusive interferometer
LW.sub.OF using the distance IL between the laser beams LWX and
LW.sub.OF. As a result, it becomes possible to use the read values
of the exposing interferometer LWX and the off-axis exclusive
interferometer LWOF as they are during exposure.
Next, the measured data D.sub.xn, D.sub.yn represents only the
relative error between the wafer coordinate system and the reticle
coordinate system. Accordingly, when the least square approximation
is performed using the wafer coordinate system as a reference, the
obtained parameters RX, Ry, .theta., .omega., Ox and Oy are all
represented as the linear errors of the reticle coordinate system
with the wafer coordinate system as the reference. With this
respect, let the x and y coordinates of the reticle coordinate
system be r.sub.xn ' and r.sub.yn ', respectively, the reticle may
be driven according to a fresh coordinates (r.sub.xn ', r.sub.yn ')
obtained, depending on the movement of the wafer coordinate system,
by the following equation: ##EQU5##
In this processing, the offsets Ox and Oy has already been
corrected at the reticle side, so that only the offset of
(<Ax>, <Ay>) is required to be corrected as the base
line amount. In addition, when the reticle coordinate system is
used as the reference, the results are all reversed and correction
may be made on the wafer coordinate system. These corrections may
be controlled separately by means of correcting on the wafer
coordinate system during the rough alignment and correcting on the
reticle coordinate system during the fine alignment.
As mentioned above, according to the present invention, the reticle
alignment and check of the base line amount are made during a
single alignment using a plurality of marks, so that it becomes
possible to average the writing error of the reticle and the
positioning error between the reticle and the wafer. This improves
the accuracy of alignment. In addition, these processes are
simultaneously performed in parallel, improving the throughput of
the operation. Further, there is no error due to the air
fluctuation of the optical path of the interferometers because the
reference mark plate 6 is applied that is capable of measuring the
reference marks at the same time in the non-scanning direction (X
direction).
However, the reference mark plate 6 moves stepwise in the scanning
direction and there may be an effect of the air fluctuation. With
this respect, the position of the wafer stage (Z.theta.-axis
driving stage 4 or the like) is locked to check the reticle
alignment and the base line amount by using the output values of
the photosensitive devices 55X and 55Y in processing with the LIA
optical system 52 in FIG. 13 for checking the base line amount.
This minimizes the effect of the air fluctuation. In addition, the
reticle marks in this embodiment are arranged at eight positions on
the four corners of the reticle 12. This is because the parameters
Rx, Ry, .theta. and .omega. are necessary as well as the offsets to
check the relation between the reticle coordinate system and the
wafer coordinate system and thus it is more advantageous to
determine the parameters Ry, .theta. and .omega. with the marks
arranged at four corners. Further, that is because, when the
reference mark plate 6 used is a light emitting type, it is
difficult to emit light from entire surface on the reference mark
plate 6 due to limitation on a light emitting portion.
In addition, let the number of the reticle marks on the reticle 12
be n, then the offset parameters OX and OY are averaged into
1/n.sup.1/2, and errors in the other parameters become small.
Accordingly, the more the number n of the reticle marks, the
smaller the error is. A simulation result on the relation among the
number n of the reticle marks, the error in the parameters and the
error in the base line amount is set forth below. In the following,
distribution at four corners on the fresh coordinate system of
(Equation 11) is represented as three times as large as a standard
deviation .sigma. with a unit of [nm].
TABLE 1 ______________________________________ Number n of Reticle
marks Error in Error in Base Worse Axis of RX, Ry, .theta., .omega.
Line Amount Square Coordinates X Y X Y Sum
______________________________________ 4 9.59 10.96 8.8 7.2 16.00 8
7.92 7.10 5.1 9.43 12 6.48 5.86 4.2 7.77 16 5.80 5.03 3.6 6.83
______________________________________
It is revealed from the above that the number n of the reticle
marks being equal to eight makes it possible to ensure the check
accuracy on the base line amount and the reticle alignment of 10 nm
or less even when the reticle writing error is 50 nm and the
stepping error of the stage is 10 nm. In other words, the higher
accuracy may be achieved by means of increasing the number n of the
reticle marks with the processing speed increased within the
limitation of the reference mark plate 6 of the light emitting
type.
In such a case, a patterning error on the reference mark plate 6
and a distortion error of the projection optical system 8 are left
as the errors in the fresh coordinate system. However, there is no
trouble at all when exposure results are compared with reference
data in adjusting the device and the results obtained are
eliminated as system offsets because these errors are hardly
fluctuated.
In the above mentioned embodiment, on the reference mark plate 6
provided are a plurality of reference marks 35A, 35B, 35C and 35D
and a plurality of reference marks 37A, 37B, 37C and 37D as shown
in FIG. 8C. However, the corresponding relation between the reticle
coordinate system and the wafer coordinate system may be obtained
by means of, for example, scanning only the reticle 12 by using a
single reference mark 35A and a single reference mark 37A, thereby
averaging or least square approximating the measured results. This
approach also contributes to reduce the effect of the writing error
of the patterns on the reticle 12.
Next, a second embodiment of the present invention is described in
conjunction with flow charts illustrated in FIGS. 15A, 15B and 16.
As for this, the reticle alignment mode in the above mentioned
first embodiment is based on the fine alignment using four pairs of
fine alignment marks 29A, 29B, 29C, 29D, 30A, 30B, 30C and 30D on
the reticle.
However, a single pair of fine alignment marks may be used for the
reticle alignment or the base line measurement after the reticle
alignment is once performed finely by means of the method described
in the first embodiment if the scaling error in the scanning
direction or the parallelism between the reticle coordinate system
and the wafer coordinate system are small. Such an alignment mode
for measuring on three items: measuring a magnification (Rx) in the
non-scanning direction, measuring rotation (.theta.) and measuring
the base line using the single pair of alignment mark is referred
to as a quick mode. In this quick mode, it is necessary to store
the writing error between the marks 29A and 30A obtained in the
fine alignment sequence to correct the writing error between the
fine alignment marks 29A and 30A.
Operation of this second embodiment is described with reference to
FIGS. 15A, 15B and 16. Operation in FIGS. 15A, 15B and 16 is the
operation of FIGS. 2A and 2B with the addition of the quick mode,
in which switching between the fine mode and the quick mode can be
available. At steps in FIGS. 15A and 15B, steps corresponding to
those in FIGS. 2A and 2B are indicated by like reference numerals,
and detailed description thereof will be omitted.
In FIGS. 15A and 15B, at steps 101 through 104, the reticle 12 is
mounted on the reticle holder and positions of the rough searching
alignment marks 27 and 28 are detected through the RA microscopes
19 and 20, respectively, as in the case of FIGS. 2A and 2B.
Subsequently, either one of the fine mode or the quick mode is
selected at step 115. The selected result is previously indicated
by an operator through the keyboard 22C in FIG. 1. It is noted that
pattern information or the like on the reticle 12 may be read by
using a bar-code reader or the like which is not shown, according
to which the main control system 22A may automatically select the
alignment mode.
When the fine mode is selected, steps 105 through 113 are executed
and the base line measurement is performed as described above by
using measured result on the reticle alignment and the fine
alignment using a plurality of fine alignment marks and a plurality
of reference marks. At step 114, obtained is the writing error
(hereinafter, referred to as "mark error") between the positions of
the actual fine alignment marks 29A and 30A relative to the target
positions on the fresh coordinate system on the reticle. The mark
error is memorized in a memorizing unit in the main control system
22A. In calculating the mark error, the reticle coordinate system
is obtained with the wafer coordinate system used as the reference
according to the relation (conversion parameters) obtained at step
113. On this reticle coordinate system, the nonlinear error is
obtained on the measured coordinate values relative to the
coordinate values in design on the fine alignment marks 29A, 29B,
29C, 29D, 30A, 30B, 30C and 30D. This nonlinear error corresponds
to the mark error. In this way, the mark error on the fresh
coordinate system on the reticle is memorized according to the
results obtained at steps 112 and 113. In addition, if the reticle
writing error is previously measured, an operator may enter the
writing error directly. When the writing error comprises a linear
component, this becomes more advantageous.
On the other hand, if the quick mode is selected at the step 115,
operation proceeds to step 116 in FIG. 16. At steps 116 through
118, the same operation is executed as in the steps 105 through 107
in FIG. 1. More specifically, images of a pair of fine alignment
marks 30A and 29A on the reticle and a pair of reference marks 36A
and 35A are observed through the RA microscopes to detect a single
reference mark 37A by using the alignment device 34 of the off-axis
type. In addition, at later half of step 119, the positions of the
marks observed through the RA microscope and the mark detected by
the alignment device 34 of the off-axis type are obtained.
Subsequently, at step 119, the mark error obtained at step 114 in
FIG. 15B is corrected relative to the position detected on the fine
alignment marks 30A and 29A on the reticle 12. As a result,
the-writing error of the patterns on the reticle 12 can be
corrected or compensated to the similar degree to the case of the
fine alignment mode in the first embodiment even if the number of
the marks measured in the quick mode is small.
Next, at step 120, the magnification error Rx in the non-scanning
direction, the rotation 0 and the offsets Ox, OY are obtained of
six conversion parameters (Rx, Ry, .theta., .omega., Ox, Oy) in
Equation 9 according to the position of each mark obtained as a
result of correction at the step 119. More specifically, as shown
in FIGS. 8A and 8C, the magnification error Rx in the non-scanning
direction is obtained from the difference between the distance
between the marks in the X direction (non-scanning direction) of
the measured reference marks 35A and 36A and the distance between
the mark images 29A and 30A in the X direction. In addition, the
rotation .theta. is obtained from a difference between a
displacement between the reference marks 35A and 36A in the Y
direction (scanning direction) and a displacement between the mark
images 29A and 30A in the Y direction and the mark distance. The
offsets Ox, OY can be given according to a mean displacement
between the mark images of the reference mark and the reticle
mark.
In the quick mode, the number of marks to be measured is two by
each at the reticle side and the reference mark plate 6 side, so
that only four out of six conversion parameters in Equation 9 are
determined. The four conversion parameters are thus obtained as
mentioned above. A scaling error Ry in the scanning direction may
be obtained by means of selecting, as the marks to be measured, two
fine alignment marks 29A and 29D aligned in the Y direction in
FIGS. 4A to 4C and two reference marks 35A and 35D in FIG. 8C.
The reticle alignment is performed according to the magnification
error Rx in the non-scanning direction, the rotation .theta. and
the offsets Ox and Oy obtained at the step 120. The measurement of
the magnification error Rx may be made by means of preparing as a
table the magnification error Rx corresponding to the difference in
the measured values on the marks from the designed values, thereby
the magnification error Rx may be obtained with the difference
between the measured values on the marks and the designed values on
the marks being applied to the table.
Next, at step 121, the base line measurement is performed by using
the measured values on the central coordinate of the reference
marks 35A and 36A as well as the measured value on the reference
mark 37A.
In this way, according to this embodiment, when the fine alignment
mode is once performed to obtain the writing error (mark error) on
the patterns of the reticle 12 and then the alignment is performed
at the quick mode, the error is corrected, so that the alignment of
the projection type exposure apparatus of the slit scanning type
can be made at a high throughput and with a high accuracy.
Next, a third embodiment of the present invention is described in
conjunction with a flow chart illustrated in FIG. 17. This third
embodiment is a case where the reticle alignment and the base line
measurement are performed in the above mentioned quick mode for
every one replacement of a predetermined number of wafers, i.e.,
for every exposure of the predetermined number of wafers. In this
embodiment, described with reference to FIG. 17 is an exemplified
operation in a case where the reticle is exchanged in the
projection type exposure apparatus in FIG. 1, following which the
patterns of the reticle 12 are successively exposed on the wafers
of which number is equal to, for example, 100.
First, at step 211 in FIG. 17, the previously used reticle is
replaced by the reticle 12 in FIG. 1 for starting the exposing
operation. In such a case, the reticle alignment and base line
check operations are performed in the quick mode that are similar
to those illustrated at steps 101 through 104 and 115 in FIG. 15A
and steps 116 through 121 in FIG. 16. Thereafter, the number of
wafers to be exposed until the next reticle alignment and the base
line check is set as an initial value of a variable N at step 212.
At step 213, the wafer is loaded on the wafer stage 4. When there
is any wafer already exposed at step 213, the exposed wafer is
first unloaded and then a new wafer is loaded.
Subsequently, at step 214, it is determined whether the variable N
is equal to zero, i.e., whether the reticle alignment and the
base-line check should be performed at that timing. If the variable
N is larger than zero, one is subtracted from the variable N at
step 215 to proceed to step 216. At the step 216, the wafer is
aligned by using the alignment device 34 of the off-axis type shown
in FIG. 13 or the alignment system of the TTL type, following which
the patterns of the reticle 12 are exposed on each shot of the
wafer. .DELTA.After completion of exposure of all (designated
number of) wafers, the exposing process on that reticle 12 is
ended. If there are one or more wafers left unexposed, the step 213
is again executed to unload the exposed wafer and load a new wafer.
This step is followed by the step 214.
If the variable N is equal to zero, i.e., whether the reticle
alignment and the base line check should be performed at that
timing at the step 214, the rotation error and the magnification
error of the reticle 12 are measured at step 217. This corresponds
to the step 120 in FIG. 16. Subsequently, step 218 is carried out
to perform the base line check in the X and Y directions of the
alignment device 34 of the off-axis type (the alignment system
comprising the FIA optical system 48 or the wafer alignment system
of a two-beam interference alignment type comprising the LIA
optical system 52). Thereafter, the number of the wafers to be
exposed until the next base line check is set as the variable N at
step 219, which returns the operation to the step 216.
As mentioned above, according to this invention, the reticle
alignment and the base line measurement are performed for every
replacement of the reticle, and the reticle alignment and the base
line measurement are performed in the quick mode for every exposure
of the predetermined number of wafers. Accordingly, it is possible
to increase an overlay accuracy between the images of the wafer and
the reticle at a high throughput.
While the technique in the above mentioned embodiment has thus been
described in conjunction with the base line measurement with the
alignment device of the off-axis type used, equivalent effects can
be obtained by applying the present invention to a TTL (through the
lens) type using within the field of the projection optical
system.
It should be understood that the present invention is not limited
to the particular embodiment shown and described above, and various
changes and modifications may be made without departing from the
spirit and scope of the appended claims.
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